Ion mobility spectrometry (IMS) is an accepted analytical method for determining the identity and concentration of trace substances present in an analyte. The basic apparatus used in the IMS process comprises an analyzer cell, a power supply furnishing accelerating and control voltages to the cell, means for ionizing samples of analyte admitted to the cell and means for determining the times required for the ions of the various substances present in the cell to traverse a specific length of the cell under the influence of an accelerating electric field and against the force of a stream of an inert drift gas flowing through the cell in the direction opposite to that of the electric field.
U.S. Pat. No. 4,633,083, issued Dec. 30, 1986, describes an Ion Mobility Spectrometer in greater detail and sets forth several conventional methods for operating the apparatus, as well as the method which is unique to the patent. The first of these methods is denominated the single scan method in which the ion entrance gate is opened for a brief period to admit a pulse of ions to the cell drift region. The small ion cloud progresses through the drift region and is separated thereby into constituent ion groups which arrive at the ion detector at different times according to the differences in mass, size and charge of the molecules of each of the groups of the constituents. Observation of the arrival times of the groups at the detector enable the identification of the molecules making up a group and measurement of the ion current resulting from the impingement of a group on the detector permits determination of the concentrations of the substances.
Another method described in the referenced patent is termed the moving second gate method. The analyzer cell used in this method includes a second ion gate, the exit gate, positioned adjacent the ion detector. The exit gate is selectively opened for a short period, usually equal to the open period of the entrance gate, to permit detection of the ions located in the near vicinity of the exit gate at the opening time. The opening of the exit gate is delayed from the opening of the entrance gate an amount of time corresponding to the time required for the ions of a particular substance to transit the cell drift region. Thus, only the ions of a particular substance will be detected for each specific delay time. By scanning the delay times, a spectrum of the substances present in the analyte may be developed.
The method of the referenced patent involves apparatus in which the analyzer cell is provided with both an ion entrance gate and an ion exit gate. Instead of delaying the opening of the exit gate from the opening of the entrance gate, as in the moving second gate method, the entrance gate and the exit gate are opened and closed simultaneously at relatively high frequencies. Such method enables the selective detection of molecules having transit times which are in phase with the gate operating frequency.